Knox, who studied business, died Wednesday evening, school officials said. The cause was not revealed, though the university said police do not suspect foul play.
"Words cannot express the heartfelt sorrow we feel because of the passing of our teammate and friend, Luke Knox," FIU coach Mike MacIntyre said in a statement. "I had the honor of coaching Luke at Ole Miss and FIU. While I admired his passion for football, his genuine love for his family and teammates is what I will always remember. He had a special way of making everyone around him feel comfortable and confident."
Knox started two games at linebacker for Ole Miss in 2019, when MacIntyre served as defensive coordinator. He played mostly special teams the past two seasons and had 11 tackles, two for loss, in 2021.
"We send our thoughts and prayers to Luke's family, friends and teammates during this difficult time," FIU athletics said in a statement. "Coaches and support staff have been reaching out to our football family and will continue to do so to ensure our student-athletes have the support they need."
FIU players were told of Knox's death late Wednesday. Thursday's practice was canceled, and grief counselors were brought in to meet with the team and others from the university community.
Ole Miss coach Lane Kiffin also offered his condolences in a tweet Thursday morning.
"I am very grateful for my personal relationship and time spent with Luke," Kiffin said. "He was a tremendous teammate and an even better person who lifted up everyone around him."
Bills coach Sean McDermott said he woke up Thursday morning to the news and emphasized that the franchise will support Dawson Knox and his family. The fourth-year tight end was not at Thursday's practice but did tweet Friday night, writing: "Our hearts are broken but we know Luke is in a much better place now with his savior Jesus."
"My heart goes out to -- our hearts go out to Dawson and his family during this time," McDermott said. "We're right there with him and supporting him and his family and just tragic news that I woke up to this morning. So we love him, and we support him and just unfortunate news this morning."
Services for Knox will take place Saturday in Tennessee, his brother said.
ESPN's Alaina Getzenberg and The Associated Press contributed to this report.